FIG. 4 shows a conventional step-up switching regulator circuit (refer to JP-2014-003850-A, for example). This switching regulator circuit is equipped with an NMOS switching transistor MN1 for voltage boosting, an inductor L1 in which energy is stored by a current that flows through it according to an input voltage Vin when the transistor MN1 is turned on, a diode D1 for charging an output capacitor C1 by a counter electromotive force current that flows from the inductor L1 when the transistor MN1 is turned off, a sense resistor Rs for detecting a current flowing through the transistor MN1, voltage division resistors Rd1 and Rd2 for generating a feedback voltage Vfb by dividing an output voltage Vout across the output capacitor C1, a switching control circuit 200 for PWM-controlling the switching of the transistor MN1 using the feedback voltage Vfb so that the output voltage Vout becomes equal to a target value and performs an overcurrent protection operation for the transistor MN1 on the basis of the voltage Vs detected by the sense resistor Rs, and a soft start circuit 300 for causing the switching regulator circuit to start operating slowly.
The switching control circuit 200 is equipped with an error amplifier 201 for generating an error voltage Verr by comparing the feedback voltage Vfb with a reference voltage Vref3 that corresponds to the target output voltage, a PWM circuit 202 for controlling the duty ratio of a PWM signal according to the error voltage Verr that is output from the error amplifier 201, a logic circuit 203 for processing the PWM signal generated by the PWM circuit 202, and a drive circuit 204 for generating a gate control voltage Vg1 according to a PWM signal that is output from the logic circuit 203 and thereby driving (i.e., switching) the transistor MN1.
The switching control circuit 200 is also equipped with an overcurrent detection comparator 205 for setting its output signal at “H” if the voltage Vs detected by the sense resistor Rs is higher than a reference value Vref1, an overcurrent detection comparator 206 for setting its output signal at “H” if the voltage Vs detected by the sense resistor Rs is higher than a reference value Vref2 (>Vref1), a counter 207 for counting the number of times the output signal of the comparator 205 becomes “H,” and an OR circuit 208 for taking the logical addition (“OR”) of the output signals of the comparator 206 and the counter 207.
The soft start circuit 300 causes an internal power source voltage to rise slowing at the time of power-on. If the output signal of the OR circuit 208 is “H,” the soft start circuit 300 stops operation of the entire switching regulator circuit by discharging a built-in capacitor (not shown).
In the switching regulator circuit shown in FIG. 4, the output voltage of the comparator 205 is set at “H” if the voltage Vs detected by the sensor resistor Rs is higher than the reference value Vref1, and the output voltages of the comparators 205 and 206 are set at “H” if the voltage Vs detected by the sensor resistor Rs is higher than the reference value Vref2. In either case, the logic circuit 203 interrupts the supply of the PWM signal to the drive circuit 204 and hence the transistor MN1 is turned off, whereby overcurrent protection is effected.
The output signal of the comparator 206 being “H” means that a large overcurrent is flowing through the transistor MN1. In this case, the built-in capacitor of the soft start circuit 300 is discharged immediately via the OR circuit 208 and operation of the entire switching regulator circuit is stopped.
The output signal of the comparator 205 being “H” means that a relatively small overcurrent is flowing trough the transistor MN1. This, in itself, does not cause discharge of the built-in capacitor of the soft start circuit 300. However, if the number of times the output signal of the comparator 205 becomes “H” has reached a prescribed number, the output signal of the counter 207 is set at “H” with a judgment that an overcurrent state is occurring continuously at times when switching is made.
Thus, the output signal of the counter 207 is changed to “H” and the built-in capacitor of the soft start circuit 300 is discharged immediately via the OR circuit 208 and operation of the entire switching regulator circuit is stopped.
However, in the switching regulator circuit shown FIG. 4, when the transistor MN1 is destroyed by short-circuiting due to an overcurrent, the result is only the logic circuit 203's stopping output of a PWM signal or discharge of the built-in capacitor of the soft start circuit 300. The short-circuit state of the transistor MN1 is maintained until operation of the entire switching regulator circuit is stopped. This protecting operation is thus incomplete.
In a case that a relatively small overcurrent is continuing, the built-in capacitor of the soft start circuit 300 is discharged if the counter has detected an output signal “H” of the comparator 205 the prescribed number of times. However, where the switching cycle of the transistor MN1 is long, it takes long time for the count of the counter 207 to reach the prescribed number, resulting in insufficient overcurrent protection. This kind of situation may occur because the switching frequency of the transistor MN1 is an item that a user can set in a desired manner.
Furthermore, the fact that the soft start circuit is used to stop operation of the entire switching regulator circuit requires a special configuration.